When drilling a well, a drill operator often wishes to deviate a wellbore or control its direction to a given point within a producing formation. This operation is known as directional drilling. One example of this is for a water injection well in an oil field that is generally positioned at the edges of the field and at a low point in that field (or formation).
To deviate a bore hole left or right, the driller may choose from a series of special downhole tools such as downhole motors, so-called “bent subs”, and steerable motors. A bent sub is a short tubular that has a slight bend to one side, is attached to the drill string, followed by a survey instrument, of which an MWD tool (Measurement While Drilling) is one generic type, followed by a downhole motor attached to the drill bit. The drill is lowered into the wellbore and rotated until the MWD tool indicates that the leading edge of the drill bit is facing in the desired direction. Weight is applied to the bit through drill collars and, by pumping drilling fluid through the drill string, the downhole motor rotates the bit.
The downhole tools communicate with equipment and controls on the surface through any suitable type of telemetry/receiver system that may both send and receive data. The telemetry/receiver system may be incorporated into the MWD tool or be a stand-alone system. Examples of such telemetry/receiver systems include wireline systems, steering tool systems, electromagnetic systems, e-line systems for pipe or coiled tubing, acoustic systems, so-called “wired pipe” systems where electric conduits are located in or in portions of the wall of the drill string, casing, or liner such as the INTELLIPIPE® by GRANT PRIDECO™, or wired composite pipe as such as the ANACONDA® by HALLIBURTON™, and mud-pulse systems where the fluid pressure in the borehole is modulated to transmit and receive data.
In addition to controlling the required drilling direction, the formation through which a wellbore is drilled exerts a variable force on the drill string at all times. This along with the particular configuration of the drill can cause the drill bit to wander up, down, right, or left. The industrial term given to this effect is “bit-walk”. The effect of bit-walk in a vertical hole can be controlled, by varying the weight on the bit of the drillstring while drilling a vertical hole. However, in a highly inclined or horizontal well, bit-walk becomes a major problem. An issue with information time delay also exists. The downhole tools used to control the drilling direction may include survey instruments attached a certain distance away from the drill bit itself, sometime by as much as thirty to forth feet. Thus, by the time the survey instruments pass the point in the wellbore where the drill bit began to change direction, the drill bit is another thirty to forty feet ahead and may have changed direction even more. Thus, there is a constant issue of inherently outdated information.
If changes in the forces that cause bit-walk occur while drilling, some tools must be withdrawn in order to correct the direction of the wellbore. The absolute requirement for tool withdrawal requires that a round trip be performed. This results in a compromise of safety and a large expenditure of time and money.
One type of drilling tool system is a rotary steerable tool (RST) that selectively controls the direction of a well bore but does not generally require withdrawal over a much broader range of changes in force that would otherwise affect the steering of the wellbore drilling with normal rotary hook up BHA assemblies. One example of an RST tool shown in FIG. 1 comprises a mandrel rotatable about a rotation axis. The rotating mandrel is used to transfer the rotary motion of the drill pipe to the drill bit and acts as continuation conduit of the drill pipe for all drilling fluids passing down the drill pipe and onto the drill bit. The system also includes a direction controller including at least an outer housing and an inner sleeve spaced apart along the mandrel. The outer housing and inner sleeve apply a force to the mandrel with a component perpendicular to the rotation axis that depends on the relative rotational position of the outer housing and the inner sleeve with respect to the mandrel. The housing has an eccentric longitudinal bore that forms a weighted side that freely rotates under gravity. The inner sleeve may also include an eccentric longitudinal bore. The apparatus also includes a driver for selectively varying the angle of the force relative to the weighted side of the housing about the rotation axis by moving the outer housing and inner sleeve independently of one another.
In operation, the driver moves the direction of the force with respect to the outer housing. A means instructs the driver to move the position of the direction of application of the force on the mandrel. Therefore, the system may further include logic means for determining when the direction of the force applied by the direction controller should be moved. The logic means may be located in the outer housing and may be configured to send and/or receive data from the surface. To communicate with the surface, the logic means may communicate with a telemetry system that is part of the bottom-hole-assembly (BHA) that in turn communicates with the surface. The communications link must allow for the relative rotation between the outer housing, the inner sleeve, and the rotating mandrel.
During assembly of the RST tool shown in FIG. 1, a sonde must be picked up to stab into and connect with the RST tool for communication. The bHA may be long and difficult to get precise distance measurements typically required for a standard electrical connection across a tool joint. For example, debris in the hang off sub may create a stand off in the expected distances as the flow tube lands on the debris instead of on the intended shoulder. Furthermore, the electrical connection can become flooded with fluid as the sonde is being lowered or raised from its position in the drill collar, creating a potential for failure via electrical shorting.